I n d . Eng. Chem. Res. 1992,31, 2635-2642
nickel content increased, the temperature where hydrogen evolved decreases. On alumina support the sol-gel processing was preferable to the impregnation in view of nanostructural control, while the impregnation was advantageous in view of catalytic performance. Based on the alumina membrane, it is important to reduce the interaction between the catalyst and the support membrane.
Acknowledgment This work was partially supported by the Ministry of Education, Science and Culture, Japan (Grant No. 01750879),and the Nippon Sheet Glass Foundation for Materials Science. Registry No. H2, 1333-74-0; MeOH, 67-56-1; Ni, 7440-02-0; A1203, 1344-28-1; MeOMe, 115-10-6; Al(OH)O, 24623-77-6.
Literature Cited Asaeda, M.; Du, L. D. Separation of Alcohol/Water Gaseous Mixtures by Thin Ceramic Membrane. J. Chem. Eng. Jpn. 1986,19, 12-17.
Leenaare, A. F. M.; Keizer, K.; Burggraaf, A. J. The Preparation and Characterization of Alumina Membranes with Ultra-Fine Pores, Part 1 Microstructural Investigations on Non-Supported Mem-
2635
branes. J . Mater. Sci. 1984, 19, 1077-1088. Leenaars, A. F. M.; Burggraaf, A. J. The Preparation and Characterization of Alumina Membranes with Ultra-Fine Porea, 2. The Formation of Supported Membranes. J. Colloid Interface Sci. 1986,105,27-40.
Mizuno, K.; Iwasaki, Y.;Hsing, C. T.; Suzuki, M. Decomposition of Methanol over Alumina Supported Rhodium Catalysta. Nenryokyokakhi 1981,60,836-841. Okubo, T.; Watanabe, M.; Kusakabe, K.; Morooka, S. Preparation of y-Alumina Thin Membrane by Sol-Gel Processing and Ita Characterization by Gas Permeation J. Mater. Sci. 1990, 25, 4822-4827.
Okubo, T.; Haruta, K.; Kusakabe, K.; Morooka, S.; Anzai, H.; Akiyama, S. Equilibrium Shift of Dehydrogenation at Short SpaceTime with Hollow Fiber Ceramic Membrane. Znd. Eng. Chem. Res. 1991,30,614-616. Okubo, T.; Watanabe, M.; Kusakabe, K.; Morooka, S. Nanoetructural Control of Sol-Gel Derived Porous Alumina via Modification of Sol. J. Mater. Sci. Lett., in press. Wu, M.; Hercules, D. M. Studies of Supported Nickel Catalysta by X-ray Photoelectron and Ion Sputtering Spectroscopies. J. Phys. Chem. 1979,83, 2003-2008. Yoldas, B. E. Alumina Sol Preparation from Alkoxides. Am. Ceram. SOC.Bull. 1975,54,28%290.
Receiued for review June 8, 1992 Accepted Auguet 28,1992
A Medium-Temperature Process for Removal of Hydrogen Sulfide from Sour Gas Streams with Aqueous Metal Sulfate Solutions Robert R. Broekhuis, David J. Koch, and Scott Lynn* Department of Chemical Engineering and Lawrence Berkeley Laboratory, Uniuersity of California, Berkeley, California 94720
A process is proposed for removing hydrogen sulfide from coal gas a t ita adiabatic saturation temperature. The coal gas would be contacted with a metal sulfate solution in a Venturi scrubber, leading
to the formation of a solid metal sulfide. This metal sulfide would be oxidized with ferric ion, forming sulfur and regenerating the metal sulfate solution. Ferrous ion would be reoxidized with air. Experimental work was done on the absorption of hydrogen sulfide into zinc and copper sulfate solutions. The rate of absorption into zinc sulfate is a strong function of pH and is unsatisfactory a t the pH proposed for this process. The rate of absorption into copper sulfate solutions is high over a large range of pH and may be modeled as an instantaneous reaction with liquid- or gas-phase mass transfer controlling. Calculations indicate that a Venturi scrubber can reduce hydrogen sulfide to a concentration below 10 ppm using a copper sulfate solution. The oxidation of zinc and copper sulfide with ferric ion at temperatures 80-150 "C yielded conversions as high as 99%; however, complete conversion of either sulfide to elemental sulfur was not achieved.
Introduction A promising method of producing clean power from coal couples coal gasification and a combustion turbine. In the coal gasification process, coal is partially oxidized with air or oxygen and reacted with water vapor at high temperature. This reaction produces a gas consisting mostly of carbon monoxide, hydrogen, carbon dioxide, and water. Under the reducing conditions of the gasifier, the sulfur content of the coal is converted to hydrogen sulfide. This is a corrosive gas that forms sulfur dioxide upon combustion. To prevent corrosion of the blades of the combustion turbine to which the coal gas is fed and to prevent emission of sulfur-containing compounds to the environment, the hydrogen sulfide must be removed before the coal gas enters the turbine. As the treatment temperature decreases, there is a loss in power plant efficiency. A hydrogen sulfide removal process operating at an elevated temperature is therefore deeirable. When the hot fuel gas is quenched with water, the loss in efficiency is minimized
since the increased mass flow due to water vaporization partially offsets the lower temperature. This paper describes a process designed to carry out the removal of hydrogen sulfide from coal gas already quenched with water to the adiabatic saturation temperature of the coal gas, typically ca. 200 O C . In this process elemental sulfur is formed as a product. In the first step of the process, the coal gas is contacted with an aqueous solution of a metal sulfate. Hydrogen sulfide is absorbed into the solution and reacta with the metal ion to form an insoluble metal sulfide. The resulting metal sulfide slurry is then reacted with ferric ion to produce sulfur and ferrous ion and to regenerate the original metal sulfate. The sulfur is separated as a salable product. The ferous ion is oxidized with air or oxygen to regenerate ferric ion. A schematic of the process is shown as Figure 1. In the first aqueous scrubber the coal gas is quenched to the adiabatic saturation temperature and particulates are removed. A substoichiometric amount of
0888-5885/92/2631-2635$03.00/00 1992 American Chemical Society
2636 Ind. Eng. Chem. Res., Vol. 31, No. 12, 1992 sour
Saturated Gas
b S
sulfur Separalloa
Regeneration Absorber
L
I
sulfur Produet
Figure 1. Flow diagram for the proposed hydrogen sulfide removal process.
ferric ion is fed to the first sulfide oxidation stage, so that the output of this stage consists of sulfur, unreaded metal sulfide, and an aqueous phase depleted of ferric ion, which can be used as absorbent in the Venturi scrubber. Ferric ion cannot be present in the scrubber since it could cause oxidation of hydrogen sulfide to sulfur, which at the absorber temperature has a nonnegligiblevapor pressure and also polymerizes to a highly viscous product. In the second oxidation atage exfemc ion is used to achieve complete sulfide conversion. Overall, the proms consumes hydrogen sulfide and oxygen and produces sulfur and water. The reactions, including the overall reaction, are shown below.
-- ++
Me2++ H2S + 2S042MeS(s) 2Fe2++ ' / 0 2
+ 2Fe3+
+ 2HS04-
Me%)
+ 2HS04-
+
(1)
Me2+ 2Fe2+ S
(2)
2Fe3+ HzO + 2S04'-
(3)
+ 1/202
S + H20 (4) For the process to be viable, the metal sulfide must form very rapidly via reaction 1and be highly insoluble. Furthermore, it must be possible to carry out reaction 2 to completion. Since both copper and zinc can be determined analytically by precipitation with hydrogen sulfide at low pH (Swift, 1949),copper and zinc sulfate were candidates for use in the process. Both metals form highly insoluble sulfides, with each of the three reaction steps being thermodynamicallyfavorable. Of the two sultides, copper sulfide is the leas soluble. The first two steps of the process were investigated experimentally for both of the metal sulfates, and that work is reported here. Previous work on the kinetics of the third reaction step by Chmielewski and Charewicz (1984) indicates that this step proceeds at a sufficiently high rate for it to be feasible in this process. H2S
Absorption of Hydrogen Sulfide into Aqueous Metal Sulfates The absorption of hydrogen sulfide into, and ita subsequent reaction with, an aqueous solution of a metal sulfate may also be represented by the reaction H2S + Me2+ MeS(s) + 2H+ (5)
-
Using published data (Weast, 1984) for the free energies and heats of formation and for the heat capacities of the reactants and products allows determining the equilibrium constant for the reaction. By applying the van't Hoff equation and assuming constant heat capacities, the effect
of temperature on the chemical equilibria can also be estimated. At 235 "C the equilibrium partial pressure of hydrogen sulfide above a solution 0.1 M in both zinc sulfate and sulfuric acid is 1.2 X lo-'' atm. For copper sulfate this equilibrium pressure is even lower, 4.0 X atm. The formation of zinc sulfide becomes thermodynamicallymore favorable as temperature increases. The reverse holds for the formation of copper sulfide. The chemical thermodynamics of the absorption and reaction are clearly sufficientlyfavorable to allow removal of hydrogen sulfide from coal gas to the desired level. Kinetics of absorption and reaction in the aqueous phase determine the practical feasibility of this process step. In gas absorption with chemical reaction, the fastest rate of absorption is obtained when the chemical reaction is instantaneous, i.e., when the gas- and liquid-phase reactants cannot coexist in solution. The rate of absorption is then limited by the rate at which the reactants and products can diffuse to and from a reaction plane in the liquid film. The resistance in the liquid phase is decreased as the liquid-phase concentration is increased, whereas the gasphase resistance remains the same. When further increases in concentration have no effect on the rate of absorption, gas-phase resistance has become limiting. The rate of absorption may be expressed in terms of a gas-based transfer coefficient by where R = rate of absorption (mol/(m3s)), a = surface area per unit volume (m2/m3),Kg = overall gas-phase masstransfer coefficient (mol/(m2Pa s)), and ApHa = driving force for mass transfer (Pa). The driving force for mass transfer is a function of the partial pressure of hydrogen s a d e , the equilibrium partial pressure of hydrogen sulfide over the absorption solution, and the absorber geometry. In absorbers in which the gas phase is well mixed, the driving force is the difference between the exit partial pressure and the equilibrium partial pressure. The absorption of hydrogen sulfide into sodium hydroxide solution is a case of absorption with instantaneous chemical reaction (Danckwerts, 1970). Therefore, absorption rates for this system are a useful point of reference for absorption into metal sulfate solutions. The product of the absorption reaction is a solid metal sulfide. The formation of solids renders many gas-liquid contacting devices, such as packed beds, impractical. An alternative contactor in which solids formation does not
Ind. Eng. Chem. Res., Vol. 31, No. 12,1992 2637
Convergng S e c t l m
Dwergng Sectman
Figure 2. Schematic diagram of Venturi scrubber.
constitute a problem is a Venturi scrubber (Figure 2). This is a gas-liquid contactor in which the liquid phase is dispersed into a high-velocity,low-pressuregas stream. The residence time of both the gas and liquid phases in the scrubber is extremely short (ca. 0.1 s), so that fast absorption kinetics of hydrogen sulfide into the liquid droplets is critical. Instantaneous reactions are most likely to be successful.
Oxidation of Metal Sulfides with Ferric Ion The second process step in the proposed process is oxidation of the metal sulfide with ferric ion, regenerating the metal sulfate solution and forming a sulfur product: MeS(s) + 2Fe3+ Me2++ 2Fe2++ S (2) Using published data (Weast, 1984) for the free energies, heats of formation,and heat capacitiesof the reactants and produds allows ascertainingthe thermodynamic feasibility of the reaction. By application of the van't Hoff equation the effect of temperature can also be determined. At 150 OC the Gibbs free energy of reaction AG, is large and negative, -121 and -57 kJ/mol for zinc and copper sulfide, respectively, so that thermodynamic limitations to this process step can be ignored. The oxidation reaction to sulfur is only a partial oxidation. Further oxidation to sulfate is possible and thermodynamically favorable. In practice it is observed that sulfur is very stable with respect to oxidation at temperatures below its melting point, due to its hydrophobic nature (Dutrizac and MacDonald, 1974). Doyle et al. (1989) remark that the oxidation overpotentialdecreases with increasing temperature, so that at 150 "C the sulfur may not be as stable. Previous work on the kinetics of oxidation of copper sulfide (Thomas and Ingraham, 1967; Mulak, 1971; Dutrizac and MacDonald, 1974) indicates that at temperatures below 80 "C the reaction is slow. The rate of reaction depends on the crystal structure and exact identity of the copper sulfide solids, with synthetic CuS showing a different behavior from the mineral covellite, which has the same composition. Formation and recovery of sulfur is seldom addressed. Crundwell(1987) proposes a model for the kinetics of zinc sulfide oxidation, based on a reaction mechanism controlled by charge transfer at the solid/liquid interface. Recovery of zinc from zinc sulfide concentrates is done hydrometallurgically by leaching at 150 OC with ferric ion as the direct oxidizing agent (Doyle et al., 1978). In this process, sulfur is recovered as a product. To prevent occlusion of the zinc sulfide particles, two organic surfactants, quebracho and calcium ligninsulfonate, are added in the leaching process. Doyle et al. (1989) propose that the additives adsorb on the mineral surface, making it more hydrophilic. It is then less likely to be wet by sulfur, so that formation of a separate sulfur phase is promoted.
-
Experimental Procedures: Absorption Experimental Apparatus. The absorption of hydrogen sulfide into aqueous solutions was studied using ap-
: t o
a
\
i
3
4 - 0
Inlet H,S
Y4
=
t=22"C
01 0
I
I
1
2
7.5% in N,
. 3
NaOH concentration (M) Figure 3. Effect of NaOH concentration on overall mass-transfer coefficient for H2S absorption.
paratus consisting of a baffled 2.8-L glans absorption vessel containing a solution of sodium hydroxide or metal sulfate, into which a mixture of hydrogen sulfide and nitrogen could be sparged. The inlet and outlet concentrations of h YH2S,out, were measured using hydrogen sulfide, y ~ g , and a Thermo Electron Model 340 H2S converter and a Thermo Electron Model 40 pulsed fluorescent SO2analyzer. The temperature inside the vessel was controlled with a water bath to a set temperature between 25 and 80 OC. Agitation was achieved by magnetic stirring at variable speeds of rotation. Absorption into Sodium Hydroxide Solution. Hydrogen sulfide, at a concentration of 0.38% in nitrogen, was absorbed into 0.1 M sodium hydroxide solution at 22 OC. The rate of absorption was very high, so that the levels of hydrogen sulfide leaving the absorber were below the detection limits of the analysis system. With a higher concentration of hydrogen sulfide, 7.5%, it was possible to determine the effects of sodium hydroxide concentration (Figure 3) and stirring rate on the overall gas-based mass-transfer coefficient. Above 0.75 M sodium hydroxide increases in sodium hydroxide concentration had no effect on the rate, indicating that the gas-phase resistance had become controlling in the absorption. Absorption into Zinc Sulfate Solution. The rate of absorption of hydrogen sulfide into zinc sulfate solutions was determined at several conditions. In these experiments, 2.5 L of the absorbent solution was used. Due to the large absorbent volume, the concentration of zinc sulfate in the absorber remained almost unchanged throughout each experiment. Mixtures of hydrogen sulfide and nitrogen were absorbed into 0.1 M zinc sulfate solution. With no added sulfuric acid, the pH of a 0.1 M zinc sulfate solution is approximately 4.8 at room temperature. The rate of absorption into such a solution was initidly high but declined as hydrogen sulfide was absorbed. The fraction of hydrogen sulfide absorbed, b~gjn - ~ ~ g , o u t ) / ~ ~ g j nis, plotted in Figure 4 against the amount absorbed for various temperatures. The rate of decline of the absorption rate became lower as the temperature increased. In actual operation of the absorption step, the pH would be in the vicinity of the bisulfate/sulfate buffer, approximately 2.0. Hydrogen sulfide gas mixtures were absorbed into solutions of zinc sulfate with the initial pH lowered by the addition of sulfuric acid. With a starting pH of 2.6
2638 Ind. Eng. Chem. Res., Vol. 31, No. 12, 1992
I
"
"
"
"
I
1.00
1 . o b
". c
t:
830
& 00000
0 0 0
a 4
9
Va
Inlet H2S
A
0.50
z
$
?I
. 0
0
0.25
7% in N,
=
t=58OC, initial pH = 2.0 0.7
G.
23OC
Inlet H# = 0.38% in N,
40°C
80°C
'
'
0.00 0.000
0.002
0.008
0.004
0.5 0.000
I 0.008
0.010
Cas Absorbed (mol/L)
Figure 4. Absorption of H8 into unbuffered 0.1 M ZnSOl solution.
I
I
I
I
0.002
0.004
0.006
0.008
H,S Absorbed (mol/L)
Figure 6. Absorption of H a into 0.1 M C&04 solution, initial pH 2.6. 10'
,
I
e
c
e
0.75
e
B 4
0.25
ii,,
I Inlet H$ = 0.38% in Nz I ZnS is first visible at -0.00028 ,
0.00 0.000
,
,
0.002
,
,
,
,
mol H,S/L. ,
,
0.004
, 0.006
,
,
,I
0.008
Gas Absorbed (mol/L)
Figure 6. Absorption of HIS into 0.1 M ZnS04 solution, initial pH 2.6.
at 60 "C,the rate of absorption is initially somewhat lower thanwith zinc sulfate solutions with no added sulfuric acid. The rate decreases rapidly until the first zinc sulfide solids become apparent. At that point, the rate of absorption increases, after which it goes through a maximum and declines in the same manner as with solutions of higher initial pH. This behavior is shown in Figure 5. When a small amount of solid zinc sulfide is added to the solution prior to an absorption run, the minimum and maximum disappear. The precipitate formed in these experiments was a very fine, white material. Absorption into Copper Sulfate Solution. The absorption rate of hydrogen sulfide into copper sulfate solutions at several conditions was determined as a function of pH and stirring intensity. The rate of absorption of hydrogen sulfide into a 0.1 M copper sulfate solution with an initial pH of 2.6 was measured at 60 "C. As with absorption into sodium hydroxide, to get a meaningful hydrogen sulfide analysis a high concentration of H2S (7%) was used. In Figure 6, the fraction of hydrogen sulfide
400
500
600
Stirrer Speed (rpm)
Figure 7. Effect of stirring rate on the overall maw-transfer coefficient for the absorption of H I S into CuSO,.
absorbed from the gas passing through the absorber is plotted versus the total amount of hydrogen sulfide absorbed. As the absorption proceeds, the pH decreases without a noticeable decrease in absorption rate. Experiments with higher initial concentrations of H2S04,up to 2.0 M, indicate that the rate of absorption remains very high even at high acidity. The effect of the stirring rate on the absorption rate was also studied. As the stirring rate increased from 400 to 600 rpm, the mass-transfer coefficient for absorption into copper sulfate solution increased dramatically. The mass-transfer coefficient for abeorption into a 2.0 M copper sulfate solution at an initial pH of 2.0, as defined in eq 6, is shown in Figure 7 as a function of stirring rate. At the higher stirring rates the mass-transfer coefficient for absorption of hydrogen sulfide into copper sulfate solutions is comparable to that for sodium hydroxide solutions (see Figure 3). The product of the reaction is a black solid with small particle size and a hydrophobic character.
Ind. Eng. Chem. Res., Vol. 31, No. 12, 1992 2639
-
I
0
P
soluble
0 insoluble
80.1'
1
I
I
I
0.2
0.3
0.4
0.5
mol Fe/kg solution -re 8. Concentration of sulfuric acid required to keep equimolar Cu/FeSO, in solution at 225 "C.
Solubility and Corrosion Studies. The conditions of the absorption step proposed for this process, an acidic aqueous solution at ca. 200 "C, are sufficiently different from commonly encountered conditions that some experimental work was done to determine solubility and corrosion properties of metal sulfate solutions at these conditions. Solutions containing equimolar amounts of copper sulfate and ferrous sulfate, varying in concentration between 0.14 and 0.54 m,were subjected to a temperature of 225 "C in a glass beaker inside a steel pressure vessel with nitrogen atmosphere. The formation of solid products, generally basic iron sulfates, depended on the amount of sulfuric acid added to the solution. The amount of acid required to keep the metal salts in solution is plotted against the amount of iron in solution in Figure 8. Coupons of various materials of construction were subjected to an aqueous solution of 0.12 M CuS04, 0.05 M FeS04, and 0.30 M H2S04at 220 "C for 6 h in the same pressure vessel. Materials tested were carbon steel, 304 stainless steel, 316 stainless steel, aluminum, and copper. None of the materials was resistant to corrosion. As may be expected from the principles governing copper winning by the cementation process with iron, cupric ions play a role in the corrosion process. In many cases specks of metallic copper were found after the experiment. Addition of copper sulfide to the corrosive solution increased the corrosion damage. Discussion: Absorption The rate-limiting step for the absorption of hydrogen sulfide into zinc sulfate solutions appears to be a crystal-growth mechanism, judging from the effect of adding solid zinc sulfide to low-pH zinc sulfate solution. The absorption rate decreases as the amount of absorbed hydrogen sulfide increases, indicating that the solution pH has a strong effect on the absorption rate. This suggests that the rate-limiting reaction is between HS- and Zn2+. On the basis of the absorption rate at temperatures up to 60 "C and at low pH, the absorption rate of hydrogen sulfide into zinc sulfate solutions is far from being instantaneous. For the purpose of removal of hydrogen sulfide from coal gas in a Venturi scrubber, the absorption rate is unfavorably low. Copper sulfate solutions have the potential of reducing hydrogen sulfide concentrations in coal gas to extremely
low levels, due to a high rate of absorption of hydrogen sulfide into these solutions. The acid concentration has a limited effect on the absorption rate. At high stirring rates, the absorption rate approaches the rate of absorption of hydrogen sulfide into sodium hydroxide solutions. The strong dependence of absorption rate on stirring intensity suggests that the formation of a solid copper sulfide film at the gas/liquid interface may limit the rate of abeorption. On the basis of experiments performed at 80 "C, the rate of reaction of hydrogen sulfide with aqueous copper ion can be modeled as instantaneous. Because of the corrosive properties of the acidic metal sulfate solution at the absorber temperature, common materials of construction are not suitable for use in the Venturi scrubber. Other corrosion-resistant alloys or titanium could be considered. Alternatively, the scrubber might be lined with a corrosion-resistant material such as glass or Teflon.
Design of Absorption Equipment A model describing the mass and heat transfer in a Venturi scrubber was developed by Uchida and Wen (1973). The model consists of differential equations describing the pressure drop, drop velocity, and mass and heat transfer in the scrubber. Important assumptions used in this model include modeling the droplets as uniformly sized rigid spheres for the purpose of estimating heat- and mass-transfer coefficients and treating the gas as an incompressible fluid. Details of the differential equations and the physical parameters used are given by Koch (1992). A computer program was developed to solve the differential equations simultaneously using the RungeKutta-Gil method. The program was used to design a Venturi scrubber capable of removing hydrogen sulfide from coal gas to a level below 10 ppm. Important parameters in Venturi scrubber design are throat diameter, liquid-to-gas ratio, and liquid-phase copper sulfate concentration. Increasing the gas velocity or the liquid-to-gas ratio increases the removal efficiency but leads to a higher pressure drop. In the suggested design, the liquid-phase concentration is set to 0.4 M, approximately the maximum value consistent with experimental solubility data. The gas velocity is chosen to be 130 m/s, which is within the range of 200-600 ft/s used in industrial Venturi scrubber applications. The liquid-bgas ratio is chosen to achieve the desired degree of hydrogen sulfide removal (to
